Electronics
Cascode Amplifier: Working Principle
A cascode amplifier stacks two transistor stages so that the first stage sees a low-impedance load.
This one design decision eliminates the Miller effect, raises output impedance by a factor of beta, and extends usable bandwidth by up to 200 times compared to a single-stage design.
This article explains exactly why that happens, with worked calculations and a live gain and bandwidth calculator.
Miller Effect Suppression
High Output Impedance
BJT and MOSFET Versions
RF and Op Amp Applications
A cascode amplifier connects two transistor stages in series: a common-emitter (or common-source) input stage followed by a common-base (or common-gate) output stage. The combination achieves voltage gain equal to a single stage but with dramatically lower input capacitance and far higher output resistance.
What Is a Cascode Amplifier?
The name comes from "cascade to cathode" -- a vacuum tube term from the 1930s. The principle transferred directly to bipolar and field-effect transistors.
In a BJT cascode, the collector of the common-emitter (CE) transistor Q1 drives the emitter of the common-base (CB) transistor Q2. The output is taken from the collector of Q2.
In a MOSFET cascode, the drain of the common-source (CS) transistor M1 drives the source of the common-gate (CG) transistor M2. The output is taken from the drain of M2.
4 pF
Typical input capacitance with cascode (vs 771 pF without)
~200x
Bandwidth improvement over single-stage CE amplifier
β × rce
Output resistance of cascode vs rce for single stage
Same
Voltage gain as a single CE stage with the same Rc
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How the Cascode Amplifier Works
The Two Stages and What Each Does
Stage 1: Common-Emitter (Q1)
Converts the input voltage to a current. Transconductance gm determines how much collector current Q1 generates per volt of input.
The voltage gain of Q1 in the cascode is very low (approximately 1) because it sees the low input impedance of Q2's emitter as its load. This is the key to eliminating the Miller effect.
Stage 2: Common-Base (Q2)
Converts the collector current from Q1 into an output voltage across the load resistor Rc. Current gain of the CB stage is approximately 1 (alpha ≈ 0.99).
The CB stage has very high output impedance, which raises the overall cascode output resistance to approximately beta times rce of Q2.
Why Stage 1 Sees Unity Gain (The Key Insight)
In a single common-emitter amplifier, the collector voltage swings by Av × Vin in the opposite direction to the base voltage.
This large swing across the base to collector capacitance Cbc is multiplied by the Miller effect, making the effective input capacitance (1 + Av) × Cbc.
In the cascode, the collector of Q1 is connected to the emitter of Q2, whose emitter impedance is approximately 1/gm2 -- very low. The voltage swing at Q1's collector is therefore tiny, approximately 1 × Vin rather than Av × Vin.
Because the Q1 collector swing is small, the Miller multiplication of Cbc is nearly eliminated. The input capacitance becomes approximately 2 × Cbc instead of (1 + Av) × Cbc.
For a CE stage with a gain of 385 V/V and Cbc of 2 pF: the Miller input capacitance is (1 + 385) × 2 = 771 pF without cascode, and approximately (1 + 1) × 2 = 4 pF with cascode. The bandwidth improvement is proportional to this capacitance ratio, giving roughly 190 times wider bandwidth.
Cascode Amplifier Key Equations
Transconductance (both BJT and MOSFET)
gm = IC / VT (BJT, at room temp VT = 26 mV)
gm = 2 × ID / (VGS - Vth) (MOSFET in saturation)
Voltage Gain (cascode, same as single CE with Rc)
Av = -gm1 × (Rc || Rout_cascode)
For Rc much smaller than Rout: Av ≈ -gm × Rc
Output Resistance (cascode -- much higher than single stage)
Rout_cascode ≈ β × rce2 (BJT cascode)
Rout_cascode ≈ gm2 × rds2 × rds1 (MOSFET cascode)
Miller Input Capacitance
Without cascode: Cin = (1 + |Av|) × Cbc
With cascode: Cin ≈ 2 × Cbc (Miller effect eliminated)
Worked Example: IC = 1 mA, β = 100, Rc = 10 kΩ, Cbc = 2 pF
gm = 0.001 / 0.026 = 38.5 mA/V
Av = 38.5 mA/V × 10 kΩ = 385 V/V (51.7 dB)
Cin without cascode = (1 + 385) × 2 pF = 771 pF
Cin with cascode = 2 × 2 pF = 4 pF -- bandwidth ×193
Cascode Amplifier Calculator
Cascode Amplifier Gain and Bandwidth Calculator
BJT cascode -- enter transistor and circuit parameters
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5 Key Advantages of the Cascode Configuration
1. Miller Effect Suppression
The CE stage sees near unity voltage gain because its collector is loaded by Q2's low emitter impedance. The base-collector capacitance is not multiplied by a large gain factor. Input capacitance falls from hundreds of picofarads to just a few picofarads.
2. Wide Bandwidth
Bandwidth is inversely proportional to input capacitance. By reducing Cin by a factor of (1 + Av), the cascode extends bandwidth by the same factor. A stage with Av = 200 extends bandwidth approximately 200 times compared to a single CE stage with the same gain.
3. High Output Impedance
The common-base output stage Q2 presents an output resistance of approximately β × rce2. This is β times higher than the rce of a single CE transistor. High output impedance makes the cascode an excellent current source and increases its voltage gain when loaded by a high-impedance current mirror.
4. Same Voltage Gain as Single Stage
Despite the bandwidth and impedance advantages, the voltage gain with a resistive load equals gm × Rc -- identical to a single CE stage. The cascode does not sacrifice gain; it improves all other parameters around the gain.
5. Improved Input to Output Isolation
The CB stage provides very low reverse transmission (Cre is tiny). The output voltage has almost no path back to the input, which improves stability, reduces feedback oscillation risk, and simplifies the design of matching networks in RF amplifiers.
MOSFET Cascode: Additional Benefit
In a MOSFET cascode, the output resistance is approximately gm2 × rds2 × rds1, which is even higher than the BJT version. This extremely high output resistance makes the MOSFET cascode the basis of regulated cascode current sources in analog integrated circuits and op amp output stages.
BJT Cascode vs MOSFET Cascode: Key Differences
| Parameter | BJT Cascode | MOSFET Cascode |
|---|---|---|
| Input stage | Common-Emitter (Q1) | Common-Source (M1) |
| Output stage | Common-Base (Q2) | Common-Gate (M2) |
| Voltage gain | Av ≈ gm × (Rc || β × rce) | Av ≈ gm × (RD || gm × rds²) |
| Output resistance | Rout ≈ β × rce2 | Rout ≈ gm2 × rds2 × rds1 (higher) |
| Miller capacitance | Cin ≈ 2 × Cbc (eliminated) | Cin ≈ Cgs + 2 × Cgd (eliminated) |
| DC bias requirement | Two VBE drops at emitter of Q2 (≈ 1.4 V headroom needed) | Two VGS drops -- more headroom lost in low supply designs |
| Noise performance | Limited by base spreading resistance, good at moderate frequencies | Very low noise at high frequencies, preferred for RF LNA design |
| Common applications | IF amplifiers, wideband instrumentation, bipolar op amp internal stages | RF LNA, CMOS op amp gain stages, analog IC current sources |
Where Cascode Amplifiers Are Used
RF Low Noise Amplifiers (LNA)
The first stage in a radio receiver must amplify the antenna signal with minimum added noise and maximum bandwidth. The cascode's low input capacitance and high reverse isolation make it the standard LNA topology from 100 MHz to millimetre wave frequencies.
Wideband Instrumentation Amplifiers
Oscilloscope front ends, spectrum analyser input stages, and test equipment preamplifiers require flat gain from DC to hundreds of megahertz. The cascode achieves this where a single CE stage would roll off at low single-digit MHz due to Miller capacitance.
Operational Amplifier Gain Stages
Regulated cascode current sources built from MOSFET cascode pairs appear inside virtually every precision CMOS operational amplifier. The very high output resistance maximises open-loop gain, which directly improves the op amp's accuracy and CMRR.
Television and Cable Tuners
Intermediate frequency (IF) amplifiers in television receivers historically used BJT cascode stages to achieve high gain at 30 to 50 MHz with stable, non-oscillating performance. MOSFET versions are used in modern cable and satellite tuner chips.
Voltage Controlled Oscillators (VCO)
The cascode is used in VCO designs where the high output impedance provides the necessary Q loading for the resonant tank circuit. The improved isolation between input and output also reduces frequency pulling effects in the oscillator loop.
Analog to-Digital Converter (ADC) Front Ends
High speed ADC input buffers use cascode stages to drive the sample-and-hold capacitor from a low-impedance source without the bandwidth penalty of Miller capacitance. The high output impedance of the cascode also reduces settling errors in switched capacitor circuits.
Advantages and Limitations at a Glance
Advantages
- Miller effect almost completely eliminated
- Bandwidth 10 to 200 times wider than a single CE stage with the same gain
- Output resistance β times higher than a single transistor
- High input to output isolation -- very stable, low oscillation risk
- Same voltage gain as a single CE stage -- no compromise on gain
- MOSFET version has extremely high output resistance suitable for precision current sources
Limitations
- Requires two transistors and more complex biasing than a single stage
- DC headroom: two VBE or VGS drops are consumed, limiting use in low supply voltage designs
- The gain is the same as a single CE stage -- cascode improves bandwidth and impedance, not gain
- At very high frequencies, the CB stage's emitter to collector capacitance limits performance
- Noise performance is dominated by the first transistor -- careful matching is needed for LNA use
Design tip: When using a BJT cascode in a signal path, bias Q2's base with a stable low-impedance voltage reference (a decoupled voltage divider or a separate regulator) rather than connecting it to a high-impedance point. Any noise or ripple on Q2's base modulates the gain directly, since the CB stage has near unity gain from emitter to collector with full transconductance from base to collector.
Watch: Cascode Amplifier and the Miller Effect Explained
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Cascode Amplifier Questions Engineers Ask
What is a cascode amplifier?
A cascode amplifier is a two-transistor circuit that connects a common-emitter (or common-source) stage directly to a common-base (or common-gate) stage. It achieves the same voltage gain as a single stage but with far lower input capacitance and much higher output resistance.
Why does a cascode amplifier have wider bandwidth?
The CE input stage sees near unity voltage gain, so the Miller effect multiplication of Cbc is almost eliminated. Input capacitance drops from (1 + Av) × Cbc to approximately 2 × Cbc, directly increasing the bandwidth by the same ratio.
What is the Miller effect?
The Miller effect multiplies the feedback capacitance Cbc by the amplifier voltage gain, making effective input capacitance much larger than the physical capacitor and limiting bandwidth. The cascode suppresses it by keeping the CE stage voltage gain near 1.
Does a cascode have more voltage gain than a single stage?
No. With a resistive load Rc, the cascode voltage gain is the same as a single CE stage: Av = gm × Rc. The advantage of the cascode is wider bandwidth and higher output impedance, not increased voltage gain.
What is the output resistance of a BJT cascode?
The output resistance of a BJT cascode is approximately β × rce2, which is β times higher than a single transistor. This makes the cascode ideal as a current source and maximises voltage gain when driving a high-impedance load.
Where is the cascode configuration used in practice?
RF low noise amplifiers, wideband instrumentation, CMOS op amp internal gain stages, VCO circuits, satellite tuners, and high speed ADC input buffers. Any application that requires high gain and wide bandwidth simultaneously benefits from the cascode topology.
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External References
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What We Learn Today
- A cascode amplifier stacks a common-emitter and a common-base stage -- the CE stage converts voltage to current, the CB stage converts that current back to voltage at the output
- The CE stage sees near unity gain in the cascode, so Miller multiplication of Cbc is eliminated -- input capacitance drops from (1 + Av) × Cbc to just 2 × Cbc
- Bandwidth improvement is proportional to the capacitance reduction -- a stage with Av = 385 achieves 193 times wider bandwidth with cascode
- Output resistance of BJT cascode is β × rce, far higher than a single transistor -- ideal for precision current sources
- Voltage gain of the cascode with resistive load equals gm × Rc -- same as single CE stage, no gain penalty
- MOSFET cascode (CS + CG) has even higher output resistance (gm × rds²) and is used in CMOS op amps and RF LNA designs
- Cascode is the standard topology for RF LNAs, wideband instrumentation, oscilloscopes, satellite tuners, and op amp gain stages
“The cascode achieves what every amplifier designer wants: high gain, wide bandwidth, and high output impedance -- the only price is one extra transistor and a few millivolts of headroom.”